A work machine, such as a track-type tractor or excavator, is typically supported and propelled by a pair of undercarriage assemblies. Each of the pair of undercarriage assemblies includes an endless track chain having a plurality of interconnected articulating components or links. Each undercarriage assembly typically also includes a drive wheel or sprocket and one or more idler wheels. The track chain is advanced around the drive sprocket and the one or more idler wheels.
During operation of the work machine, it is necessary to maintain tension on the track chain in order to keep the chain from derailing from the drive wheel or sprocket or the idler wheel. In order to maintain tension on the track chain, a tension adjustment mechanism such as a hydraulic cylinder or coiled spring is often included in the undercarriage assembly.
With regard to excavators, it is generally desirable to have the track chain relatively taut during performance of a digging or other type of work operation in order to prevent wear on the components associated with the undercarriage assembly and to provide a more stable work platform. A relatively taut track chain helps prevent the excavator from rolling back and forth within the interior of the track chain as a result of recoil forces generated during performance of the work operation. Hence, a relatively high tension level is desirably maintained on the track chains of excavators during a work operation, even though it is known that use of such a high tension level increases the rate at which components associated with the undercarriage assembly wear, especially during travel. To create tension on the track chain, the hydraulic cylinder or the coiled spring of the tension adjustment mechanism urges the idler wheel away from the drive wheel or sprocket, increasing the dimension of the undercarriage assembly which the track chain must encircle.
In contrast, it is generally desirable to have the track chain relatively loose during advancement or travel of an excavator. By loosening or otherwise decreasing tension on the track chain, wear on the components associated with the undercarriage assembly is reduced. This increases the efficiency and even the useful life of the excavator. To reduce tension in the track chain, the hydraulic cylinder or coiled spring of the tension adjustment mechanism moves the idler wheel toward the drive wheel or sprocket a certain incremental amount.
The tension adjustment mechanism also provides a recoil function in the track chain, accommodating temporary forces on the track such as when a rock or the like is ingested between the track and the wheels during advancement of the excavator. In these instances, the idler wheel is permitted to recoil toward the drive sprocket in order to accommodate the extra length the track must encircle in order to accommodate the rock without breaking.
When traveling in one direction, the rotation of the drive wheel or sprocket pulls on the top flight of the track chain, exerting a recoil force on the idler wheel. During travel in the opposite direction, the drive wheel or sprocket pulls on the bottom flight of the track chain which, because the bottom flight bears the weight of the machine against the ground, does not significantly transmit that force through the track chain to the idler wheel. Conventional track tension/recoil systems have a tensioning mechanism for varying track tension based on dig and travel requirements, and accordingly are not optimized for either.
U.S. Pat. No. 6,249,994 (“the '994 patent”) discloses a tensioning mechanism which decreases track tension by a predetermined amount when the machine is traveling. The '994 patent discloses detecting an increase in drive pressure when the machine goes into its travel mode (because fluid is diverted from a work implement to the drive system) and then decreasing tension on the track chain in response to the detection of the increase in fluid drive pressure. Conversely, the '994 patent discloses detecting a decrease in drive pressure when the machine goes into its work mode (because fluid is diverted from the drive system to the work implement) and then increasing tension on the track chain in response to the detection of the increase in fluid drive pressure. The '994 patent does not disclose, however, differentiating between directions of travel and the drive force required to achieve travel.
In addition, conventional work machines do not effectively compensate for the advancement of the excavator under high drawbar loads, which is a common cause of “sprocket jumping,” a condition where a bushing on the track chain slips over a tooth on the drive sprocket. Sprocket jumping has a detrimental effect on the wear of the components of the undercarriage assembly and on the hydraulic drive motor as it reflects large spikes in the pressure going into the drive motor. In addition, the recoil force created by the drive sprocket on the idler wheel during travel in one direction increases in proportion to the energy required to move the machine (referred to as drawbar load), sometimes moving the idler wheel too close to the drive sprocket under high drawbar loads.
The present invention is directed to solving one or more of the problems or disadvantages set forth above of current work machines.